article: When the Audet family turns on the lights at Blue Farm, they are using electricity that comes from cows--cow manure  , to be specific.
  Cows produce a lot of wastes. One cow can create 30 gallons of wastes each day. Now imagine the output of over 1,000 cows at Blue Farm. That's really a big amount.
  When farmers clean their barns, they put the wastes in a big heap, and spread some of them on their fields for fertilizer . But now places like Blue Spruce Farm have a new way of using cow wastes. They use them to produce electricity.
  Here's how it works: A big machine moves back and forth cleaning the barn floor. "The cows aren't bothered," says Marie Audet. "They are animals of habit; they get used to it, and just lift one foot and then another to let it go by."
  The machine pushes the manure into a big 600-gallon tank like a small swimming pool. The tank is called a digester   because what happens there is just like what happens inside a cow: Bacteria  get to work and continue to digest the manure.
  Methane gas (, )in the atmosphere is known as a "greenhouse" gas because it keeps heat just like a greenhouse does, causing our planet to warm up. That's an environmental concern. But the digester has a good result. Its gas is kept and used to power electric generators  .
  At Blue Farm, the generators make enough electricity to power 400 homes. The Audet family sells the extra electricity they can't use themselves.
question: According to the passage, which of the following is changed into electricity?
options: The power of moving water. The heat of burning coal. The power of sunlight. The heat of the methane gas